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大槻 太毅*; Ishida, Tatsuhiro*; Tsutsumi, Naoya*; 小林 正起*; Inagaki, Kodai*; 吉田 鉄平*; 竹田 幸治; 藤森 伸一; 保井 晃*; 北川 彩貴*; et al.
Physical Review Materials (Internet), 7(12), p.124601_1 - 124601_6, 2023/12
被引用回数:0 パーセンタイル:0.00(Materials Science, Multidisciplinary)InMn
Sn
O
is a diluted magnetic semiconductor combining transparent, ferromagnetism, and semiconducting properties. We report the electronic structure of In
Mn
Sn
O
(x = 0.0, 0.2, and 0.3) by means of hard X-ray photoemission spectroscopy (HAXPES), soft X-ray resonant photoemission spectroscopy (RPES), and X-ray absorption spectroscopy (XAS). The spectral shape of the Mn
-
XAS spectra indicates that the valence of the substitutional Mn ions is divalent. The In 3
and O
core-level shifts with Mn substitution suggest the hole doping due to the Mn
ions. The valence-band spectra exhibit the well-defined features associated with the donor states across the Fermi level (EF) and the valence-band edge. The valence-band maximum (VBM) shifts to higher binding energy and the spectral weight near EF decreases with increasing Mn concentration, which is consistent with the hole doping nature observed in the core-level shift. The Mn
-3
RPES reveals that the hump structure around 1.9 eV above the VBM is originating from the Mn 3
impurity band and the valence-band state consists of the O 2
band strongly hybridized with the Mn 3
orbital. Furthermore, there is no contribution of the Mn 3
orbital to the spectral weight close to
. Based on our results, we argue the possible mechanism of the ferromagnetism for In
Mn
Sn
O
.
藤森 伸一; 川崎 郁斗; 竹田 幸治; 山上 浩志; 雀部 矩正*; 佐藤 芳樹*; 清水 悠晴*; 仲村 愛*; Maruya, A.*; 本間 佳哉*; et al.
Electronic Structure (Internet), 5(4), p.045009_1 - 045009_7, 2023/11
The electronic structure of the -based superconductor
was studied by photoelectron spectroscopy. The energy distribution of the
state was revealed by the
resonant photoelectron spectroscopy. The
state was mostly distributed in the vicinity of the Fermi energy, suggesting the itinerant character of the
state. The contribution of the
state to the density of states (DOS) at the Fermi energy was estimated to be nearly half of that of the
states, implying that the
state has aconsiderable contribution to the DOS at the Fermi energy. The
core-level and
X-ray absorption spectra were analyzed based on a single-impurity Anderson model. The number of the
state in the ground state was estimated to be 0.8-0.9, which is much larger than the values obtained in the previous studies (i.e., 0-0.4).
川崎 郁斗; 竹内 一陽*; 藤森 伸一; 竹田 幸治; 山上 浩志; 山本 悦嗣; 芳賀 芳範
Physical Review B, 108(16), p.165127_1 - 165127_9, 2023/10
We performed angle-resolved photoemission spectroscopy (ARPES) experiments using soft X rays to investigate the electronic structure of the heavy-fermion superconductor UPt. The overall band structure revealed by the present ARPES measurements is compared with density-functional calculations for UPt
and the non-
reference compound ThPt
. We showed that the calculation for ThPt
gives a better description of the experimental band structure, except for the
-derived heavy quasiparticle bands. This situation is reminiscent of Ce-based heavy-fermion systems, whose band structures are often very similar to those of non-
La-based compounds. The narrow heavy quasiparticle bands are located just below the Fermi level (
), and their band structure could not be resolved by the present energy resolution. We also showed that most of the U
spectral weight exists not as the coherent heavy-fermion bands but as incoherent components that distribute over a wide energy range, spanning from near
to approximately 2 eV. This result indicates that the heavy quasiparticle bands are enormously renormalized by the electron correlation effect. Additionally, we showed that the spectral intensity of the heavy-fermion bands is almost temperature independent, at least up to 100 K, where the magnetic susceptibility follows a Curie-Weiss behavior.
竹田 幸治; Pospil, J.*; 山上 浩志; 山本 悦嗣; 芳賀 芳範
Physical Review B, 108(8), p.085129_1 - 085129_10, 2023/08
被引用回数:0 パーセンタイル:0.00(Materials Science, Multidisciplinary)We investigated the element-specific electronic states and magnetic properties of ferromagnetic superconductor UCoGe and Ru-substituted U(CoRu
)Ge using soft X-ray magnetic circular dichroism (XMCD) experiments. YCoGe, a nonmagnetic compound without the 5
electrons, was also examined as a reference material. The shapes of the XMCD spectra of UCoGe at the U
and Ge
edges are indistinguishably similar to those of U(Co
Ru
)Ge. On the other hand, the XMCD spectral shape at the Co
edge is very peculiar and is changed by the existence of the U 5
electrons and the Ru substitution. From the sum rule analysis for the XMCD spectra, it was found that the magnetic moments of the U 5
and Co 3
electrons in UCoGe and U(Co
Ru
)Ge are aligned in the same direction under the present experimental conditions;
= 5.5 - 50 K and
= 0 - 10 T. Furthermore, even in the case of YCoGe, the Co 3
electrons have a large orbital magnetic moment, suggesting that the Co 3
electrons in these compounds are in a low symmetric electronic environment originally. Through the
- and
-dependence of the XMCD intensity, it was revealed that the magnetic properties at the Co site are not simply induced by the magnetism at the U site and that the Co 3
electrons play an important role in the stability of ferromagnetism.
川崎 郁斗; 藤森 伸一; 竹田 幸治; 山上 浩志; 大貫 惇睦*
Journal of the Physical Society of Japan, 92(6), p.064709_1 - 064709_8, 2023/06
The electronic state of 4 localized ferromagnet CeRu
Ge
has been investigated by angle-resolved photoemission spectroscopy (APRES) experiments using soft X rays. The band structure and Fermi surfaces consisting of non-4
bands are almost consistent with a calculation based on the density-functional theory for LaRu
Ge
, thus confirming a localized character of the 4
electrons. On the other hand, the Ce 3
-4
resonant ARPES experiments demonstrated that the 4
spectral function is dominated by
components in the vicinity of the Fermi level and thus is different from that expected for a localized electron system. We also revealed that the intensities of the
components have almost no temperature dependence up to at least 100 K, which is two orders of magnitude larger than the Kondo temperature of CeRu
Ge
.
Wang, Y.*; 梶原 駿*; 松岡 秀樹*; Saika, B. K.*; 山神 光平*; 竹田 幸治; 和達 大樹*; 石坂 香子*; 岩佐 義宏*; 中野 匡規*
Nano Letters, 22(24), p.9964 - 9971, 2022/12
被引用回数:15 パーセンタイル:90.35(Chemistry, Multidisciplinary)In a conventional magnetic material, a long-range magnetic order develops in three dimensions, and reducing a layer number weakens its magnetism. Here we demonstrate anomalous layer-number-independent ferromagnetism down to the two-dimensional (2D) limit in a metastable phase of CrTe
. We fabricated Cr
Te
thin films by molecular-beam epitaxy and found that Cr
Te
could host two distinct ferromagnetic phases characterized with different Curie temperatures (
). One is the bulk-like high-
phase showing room-temperature ferromagnetism, which is consistent with previous studies. The other is the metastable low-
phase with
160 K, which exhibits a layer-number-independent
down to the 2D limit in marked contrast with the conventional high-
phase, demonstrating a purely 2D nature of its ferromagnetism. Such significant differences between two distinct phases could be attributed to a small variation in the doping level, making this material attractive for future ultracompact spintronics applications with potential gate-tunable room-temperature 2D ferromagnetism.
角田 一樹; 日下 翔太郎*; 竹田 幸治; 小林 功佳*; 平原 徹*
Physical Review B, 106(19), p.195421_1 - 195421_7, 2022/11
被引用回数:1 パーセンタイル:14.73(Materials Science, Multidisciplinary)We systematically investigated the electronic properties of monolayer and multilayer vanadium selenide films grown with different conditions using in situ angle-resolved photoemission spectroscopy and first-principles calculations. We observed the band dispersions of pristine VSe in a sample grown at substrate temperature (
) of 250
C, while the significant modulations of the Se-derived bands can be seen at
= 380
C. For
= 420
C, we found that the V self-intercalated V
Se
was formed. As a simple and convenient way, we also demonstrate that the monolayer V
Se
can be fabricated by annealing from the multilayer VSe
film, and the possible formation mechanism is discussed. The establishment of the growth methods of V
Se
film and verifying its electronic properties provide the substantial step for the creation of the 2D vDW heterostructures.
山神 光平*; 藤澤 唯太*; Pardo-Almanza, M*; Smith, B. R. M.*; 角田 一樹; 竹田 幸治; 岡田 佳憲*
Physical Review B, 106(4), p.045137_1 - 045137_8, 2022/07
被引用回数:2 パーセンタイル:30.40(Materials Science, Multidisciplinary)FeGeTe
is a van der Waals (vdW) coupled unconventional ferromagnetic metal with a high Curie temperature (
), exceeding 300 K. The formation of an anomalous ground state far below
has attracted great interest, leading to a need to understand the spin-polarized electronic state near the Fermi energy (
) at low temperature. Despite recent extensive study, a microscopic understanding of the spin polarized electronic structure around
has not been established yet due to the intrinsic complexity of both the crystal and band structures. In this work, we investigate the temperature dependence of element-specific soft X-ray magnetic circular dichroism (XMCD). Systematic temperature evolution in the XMCD signal from both magnetic Fe and its ligand Te is clearly observed. Importantly, the enhancement of the hybridization between Fe 3
and Te 5
states in the zero-magnetic field limit has been revealed. In light of the possible emergence of an exotic ground state, we discuss the temperature and magnetic field evolution of the element specific XMCD results.
窪田 崇秀*; 高野 大地*; 小田 洋平*; Mohanty, S.*; 伊藤 啓太*; 松木 充弘*; 林田 誠弘*; Sun, M.*; 竹田 幸治; 斎藤 祐児; et al.
Physical Review Materials (Internet), 6(4), p.044405_1 - 044405_12, 2022/04
被引用回数:5 パーセンタイル:54.89(Materials Science, Multidisciplinary)This study focuses on an inverse-type Heusler alloy, MnCoGa
(MCG), with a small off-stoichiometry (
), showing perpendicular magnetic anisotropy (PMA). We observed a relatively large uniaxial magnetocrystalline anisotropy constant (
) on the order of 10
J/m
at room temperature in the MCG films with a small tetragonal distortion of a few percent. X-ray magnetic circular dichroism (XMCD) was employed to get insight into the origin for the PMA. Negligible angular variation of the orbital magnetic moment evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to
. The origin of the PMA was discussed based on the second-order perturbation theory in terms of the spin-orbit coupling, and first principles calculations, claiming that the mixing of the occupied
- and the unoccupied
-spin states is responsible for the PMA of the MCG films.
藤森 伸一; 竹田 幸治; 山上 浩志; Pospil, J.*; 山本 悦嗣; 芳賀 芳範
Physical Review B, 105(11), p.115128_1 - 115128_6, 2022/03
被引用回数:0 パーセンタイル:0.00(Materials Science, Multidisciplinary)The electronic structure of , which is isostructural to the heavy fermion superconductor
, was investigated by photoelectron spectroscopy. The band structure and Fermi surfaces of
were obtained by angle-resolved photoelectron spectroscopy (ARPES), and the results were well-explained by the band-structure calculation based on the local density approximation. The comparison between the ARPES spectra and the band-structure calculation suggests that the Fermi surface of
mainly consists of the
and
states with a minor contribution from the
states. The comparison of the band structures between
and
argues that the
states form Fermi surfaces in
through hybridization with the
state in the
layer, suggesting that the Fermi surface of
has a strong three-dimensional nature.
角田 一樹; 竹田 幸治; 日下 翔太郎*; 小林 功佳*; 平原 徹*
Physical Review Materials (Internet), 6(1), p.014006_1 - 014006_8, 2022/01
被引用回数:6 パーセンタイル:61.59(Materials Science, Multidisciplinary)二次元強磁性体候補物質である単層1-VSe
薄膜の磁気特性を、元素選択的磁気プローブであるX線磁気円二色性(XMCD)を用いて調べた。高分解能測定を行うことで、外部磁場下において原子レベルの薄さの1
-VSe
薄膜から明確なXMCDシグナルを検出することに成功した。X線の入射角度を操作することで、面内と面外の磁気特性を分離し、強い磁気異方性を見出すことができた。さらに、磁場と温度に依存したXMCD測定により、6Kでも長距離強磁性秩序は存在せず、隣接するバナジウムイオン間の短距離強磁性・反強磁性相互作用が存在することが明らかになった。このような低温での磁気挙動は、単層1
-VSe
が強磁性になりかけていることを意味しており、VSe
系ヘテロ構造で報告されている近接効果誘起強磁性の説明となる。
川崎 郁斗; 小畠 雅明; 藤森 伸一; 竹田 幸治; 山上 浩志; 辺土 正人*; 仲間 隆男*; 大貫 惇睦*
Physical Review B, 104(16), p.165124_1 - 165124_8, 2021/10
被引用回数:0 パーセンタイル:0.00(Materials Science, Multidisciplinary)We carried out angle-resolved photoemission (ARPES) experiments using soft X rays to investigate the electronic structure of the intermediate valence compound EuNiP
. Both the Eu
and Eu
components arising from the 4
and 4
final states were observed in the valence spectra, directly confirming an intermediate character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and we found that the ARPES spectra up to just below the Fermi level can well be explained by a calculation based on the density-functional theory for the non-4
reference compound SrNi
P
. The heavy-fermion bands in EuNi
P
are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for SrNi
P
, and the Eu
components located at the very vicinity of the Fermi level.
藤原 秀紀*; 梅津 理恵*; 黒田 文彬*; 宮脇 淳*; 樫内 利幸*; 西本 幸平*; 永井 浩大*; 関山 明*; 入澤 明典*; 竹田 幸治; et al.
Scientific Reports (Internet), 11(1), p.18654_1 - 18654_9, 2021/09
被引用回数:0 パーセンタイル:0.00(Multidisciplinary Sciences)Band-gap engineering is one of the fundamental techniques in semiconductor technology. To fully utilize the spintronic material, it is essential to optimize the spin-dependent electronic structure in operando conditions by applying the magnetic and/or electric fields. Here we present a new spectroscopic technique to probe the spin-polarized electronic structures by using magnetic circular dichroism (MCD) in resonant inelastic soft X-ray scattering (RIXS) under an external magnetic field. Thanks to the spin-selective dipole-allowed transitions in the RIXS-MCD, we have successfully demonstrated the direct evidence of the perfectly spin-polarized electronic structures for the prototypical halfmetallic Heusller alloy, CoMnSi. The RIXS-MCD is a promising tool to probe the spin-dependent carriers and band-gap with element specific way induced in buried magnetic layers under operando conditions.
窪田 崇秀*; 嶋田 雄介*; 土屋 朋生*; 吉川 智己*; 伊藤 啓太*; 竹田 幸治; 斎藤 祐児; 今野 豊彦*; 木村 昭夫*; 高梨 弘毅*
Nanomaterials (Internet), 11(7), p.1723_1 - 1723_11, 2021/07
被引用回数:2 パーセンタイル:16.40(Chemistry, Multidisciplinary)Heusler alloys exhibit various magnetic properties. In this study, a layered sample consisting of a Heusler alloy, MnVAl and a ferromagnet, Fe, is selected as a material system exhibiting exchange bias. Although the fully ordered Mn
VAl is known as a ferrimagnet, the Mn
VAl/Fe layered structure exhibits exchange bias. The high-angle annular dark field STEM images demonstrated the formation of Mn
VAl clusters with the L2
phase distributed only around the interface to the Fe layer in the sample. Furthermore, the antiferromagnetic coupling between the Mn- and Fe-moments were observed in element specific hysteresis loops measured using XMCD. The locally ordered L2
phase and antiferromagnetic Mn-moments in the Mn
VAl layer are important for the exchange bias.
藤森 伸一; 竹田 幸治; 山上 浩志; 山本 悦嗣; 芳賀 芳範
Electronic Structure (Internet), 3(2), p.024008_1 - 024008_8, 2021/06
The three-dimensional (3D) electronic structure of the hidden order compound in a paramagnetic phase was revealed using a 3D angle-resolved photoelectron spectroscopy where the electronic structure of the entire Brillouin zone is obtained by scanning both incident photon energy and detection angles of photoelectrons. The quasi-particle bands with enhanced contribution from the
state were observed near
, formed by the hybridization with the
states. The energy dispersion of the quasi-particle band is significantly depend on
, indicating that they inherently have a 3D nature. The band-structure calculation qualitatively explain the characteristic features of the band structure and Fermi surface although the electron correlation effect strongly renormalizes the quasi-particle bands. The 3D and strongly-correlated nature of the quasi-particle bands in
is an essential ingredient for modeling its hidden-order transition.
小林 正起*; Anh, L. D.*; 鈴木 雅弘*; 金田-高田 真悟*; 竹田 幸治; 藤森 伸一; 芝田 悟朗*; 田中 新*; 田中 雅明*; 大矢 忍*; et al.
Physical Review Applied (Internet), 15(6), p.064019_1 - 064019_10, 2021/06
被引用回数:5 パーセンタイル:42.31(Physics, Applied)A fundamental understanding of the interfacial magnetic properties in ferromagnetic heterostructures is essential for utilizing ferromagnetic materials for spintronic device applications. Here, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO(LAO)/La
Sr
MnO
(LSMO)/Nb:SrTiO
(Nb:STO) heterostructures with varying LSMO layer thicknesses, in which the magnetic anisotropy strongly changes with the LSMO thickness due to the delicate balance between strains originating from both the Nb:STO and LAO layers, using X-ray magnetic circular dichroism and photoemission spectroscopy. We successfully detect the clear change of the magnetic behavior of the Mn ions concomitant with the thickness-dependent metal-insulator transition. Our results suggest that the double-exchange interaction induces ferromagnetism in the metallic LSMO film under tensile strain caused by the STO substrate, while the superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. The change in strain, depending on LSMO layer thickness, is confirmed by scanning transmission electron microscopy. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via Mn
orbitals under compressive strain and the double-exchange interaction via the
orbitals. These findings provide key aspects of ferromagnetic oxide heterostructures for the development of spintronic device applications.
深澤 拓朗*; 日下 翔太郎*; 角田 一樹; 橋爪 瑞葵*; 一ノ倉 聖*; 竹田 幸治; 出田 真一郎*; 田中 清尚*; 清水 亮太*; 一杉 太郎*; et al.
Physical Review B, 103(20), p.205405_1 - 205405_6, 2021/05
被引用回数:9 パーセンタイル:63.89(Materials Science, Multidisciplinary)We successfully fabricated a MnBiTe
/Bi
Te
heterostructure by incorporating Mn and Te inside the topmost quintuple layer of Bi
Te
, as unambiguously confirmed by LEED I-V scanning transmission electron microscopy measurements. The surface-state Dirac cone of the heterostructure showed little change compared to that of the pristine Bi
Te
and X-ray magnetic circular dichroism measurements showed that the system was paramagnetic down to 5.6 K. These results are in contrast to the previous works on related materials that showed magnetic order around 10 K as well as theoretical predictions and suggests the intricacy of the magnetic properties of two-dimensional van der Waals magnets.
松岡 秀樹*; Barnes, S. E.*; 家田 淳一; 前川 禎通; Bahramy, M. S.*; Saika, B. K.*; 竹田 幸治; 和達 大樹*; Wang, Y.*; 吉田 訓*; et al.
Nano Letters, 21(4), p.1807 - 1814, 2021/02
被引用回数:14 パーセンタイル:77.78(Chemistry, Multidisciplinary)Magnetocrystalline anisotropy, a key ingredient for establishing long-range order in a magnetic material down to the two-dimensional (2D) limit, is generally associated with spin-orbit interaction (SOI) involving a finite orbital angular momentum. Here we report strong out-of-plane magnetic anisotropy without orbital angular momentum, emerging at the interface between two different van der Waals (vdW) materials, an archetypal metallic vdW material NbSe possessing Zeeman-type SOI and an isotropic vdW ferromagnet V
Se
. We found that the Zeeman SOI in NbSe
induces robust out-of-plane magnetic anisotropy in V
Se
down to the 2D limit with a more than 2-fold enhancement of the transition temperature. We propose a simple model that takes into account the energy gain in NbSe
in contact with a ferromagnet, which naturally explains our observations. Our results demonstrate a conceptually new magnetic proximity effect at the vdW interface, expanding the horizons of emergent phenomena achievable in vdW heterostructures.
藤森 伸一; 川崎 郁斗; 竹田 幸治; 山上 浩志; 仲村 愛*; 本間 佳哉*; 青木 大*
Journal of the Physical Society of Japan, 90(1), p.015002_1 - 015002_2, 2021/01
被引用回数:13 パーセンタイル:79.65(Physics, Multidisciplinary)The valence state of was studied by core-level photoelectron spectroscopy. The main peak position of the
core-level spectrum of
coincides with that of
, which is anitinerant compound with a nearly
configuration. However, the main peak of
is broader than that of
, and satellite structures are observed in the higher binding energy side of the main peak, which are characteristics of mixed-valence uranium compounds. These results suggest that the
state in
isin a mixed valence state with a dominant contribution from the itinerant
configuration.
竹田 幸治; 大矢 忍*; Pham, N. H.*; 小林 正起*; 斎藤 祐児; 山上 浩志; 田中 雅明*; 藤森 淳*
Journal of Applied Physics, 128(21), p.213902_1 - 213902_11, 2020/12
被引用回数:8 パーセンタイル:47.37(Physics, Applied)In order to understand the mechanism of the ferromagnetism in GaMn
As ((Ga,Mn)As), we have investigated the magnetic behavior on a microscopic level through systematic temperature (
) and magnetic-field (
) dependent soft X-ray magnetic circular dichroism (XMCD) experiments at the Mn
absorption edges. The
and
dependences of XMCD intensities have been analyzed using a model consisting of the ferromagnetic (FM), paramagnetic, and superparamagnetic (SPM) components. Intriguingly, we have found a common behavior for the ferromagnetic ordering process in (Ga,Mn)As samples with different Mn concentrations and different Curie temperature (
) values. In particular, the SPM component develops well above
, indicating that local FM regions are formed well above
. The present findings indicate that the onset of ferromagnetic ordering is triggered by local electronic states around the substitutional Mn ions. Insight into the most representative ferromagnetic semiconductor, (Ga,Mn)As, will be an important step in understanding the mechanism of ferromagnetic ordering in various ferromagnetic semiconductor families.